1. Field of the Invention
The present invention generally relates to optical write apparatuses and, more particularly, to an optical write apparatus for use in a printer, a digital copier or a laser facsimile machine and applicable to a measuring instrument and a display using laser beam scanning.
2. Description of the Related Art
In laser printers, digital copiers, laser facsimile machines and the like, there is used an optical write system for forming an image by scanning a photosensitive body with a laser beam emitted by a semiconductor laser or the like and deflected by a beam deflector such as a polygon mirror. For example, Japanese Laid-Open Patent Application No. 2-29614 discloses such an optical write apparatus.
The optical write apparatus disclosed in Japanese Laid-Open Patent Application No. 2-29614 is a two-beam optical write apparatus. In this two-beam optical write apparatus, two polarized laser beams having respective planes of polarization at right angles to each other are combined using a deflecting beam splitter. The two beams exiting the deflecting beam splitter are then transformed into beams circularly polarized in opposite directions by passing through a quarter-wave plate. The circularly polarized beams are deflected by a beam deflector embodied by a polygon mirror so as to scan a scanned surface. Due to circular polarization, the two beams have identical polarizations at the deflecting surface of the beam deflector even if they differ in the vector rotation direction. Therefore, the beam deflector reflects the two beams with substantially the same reflectivity so that the entire optical system may have substantially uniform transmittivity with respect to the beams.
Generally, a photosensitive body of a write optical apparatus has uneven photosensitivity. For reasons derived from the production process, unevenness usually occurs at the ends of the photosensitive body in the scanning direction. A background for this unevenness will be discussed.
A photosensitive (OPC) drum is usually produced using a so-called dipping process. FIGS. 1A-1C are schematic diagrams showing the dipping process. As shown in FIG. 1A, a drum 11 is dipped in a solution 27 of a photosensitive material (OPC) filling a container such that a rotation axis of the drum 11 is parallel with the longitudinal axis of the container. As shown in FIG. 1B, a photosensitive film is formed on the drum 11 by raising the drum 11 from the solution 27. As indicated by a curve 28 of FIG. 1B, the resultant photosensitive drum 11 produced by the dipping process is uneven in thickness of the photosensitive film. More specifically, after the drum 11 is raised from the solution 27, the solution 27 is drawn toward the lower end of the drum 11 due to gravity action, resulting in a relatively small film thickness toward the higher end of the drum 11 and a relatively large film thickness toward the lower end thereof.
In the presence of unevenness in thickness of the photosensitive film on the drum 11, uneven photosensitivity distribution on the drum 11 results. As indicated by a curve 29 of FIG. 1C, photosensitivity on the drum 11 is relatively low toward the higher end of the drum 11 where the photosensitive film is relatively thin, and is relatively high toward the lower end of the drum 11 where the photosensitive film is relatively thick.
Photosensitivity of the surface of the drum 11, exposure of the surface of the drum 11 to a laser beam (light intensity of the laser beam) and the amount of charge built up on the surface of the photosensitive drum 11 are generally bound in a relationship EQU photosensitivity!.times.exposure (light intensity)!.varies.amount of charge!
Assuming that the light intensity of the laser beam scanning the photosensitive body remains unchanged over the scanning range, uneven photosensitivity distribution on the drum 11 results in a relatively large amount of charge built up in a portion with high photosensitivity and a relatively small amount of charge built up in a portion with low photosensitivity.
As a result, there is produced unevenness in image density. One approach to avoid the unevenness is to fit a charger for uniformly charging the surface of the photosensitive drum 11 at an angle with respect to the rotation shaft of the photosensitive drum 11 so that the charger has varying gaps with respect to the surface of the photosensitive drum 11 depending on the level of photosensitivity on the surface of the photosensitive drum 11. In addition, the gaps could be adjusted so as to ensure that charge is built up evenly on the surface of the photosensitive drum 11.
Usually, a motor is used to drive a beam deflector embodied by a polygon mirror. A problem presented by noise produced by the motor is generally resolved by accommodating the beam deflector in a housing. The portion of such a housing where the laser beam passes should be formed of a transparent member in order to ensure a desired level of light intensity. The light intensity of the beam on the photosensitive drum 11 varies significantly depending on the angle of incidence of the laser beam on the transparent member. As a result, even when it is assumed that the photosensitive drum 11 has an even photosensitivity distribution, the amount of charge built up on the photosensitive drum 11 is not even.